The present invention relates generally to audio signal processing circuitry and more particularly to a "direct box" circuit for coupling a high impedance audio source to a low impedance audio destination.
Many popular musical instruments today have a pickup or transducer by which the sound of the instrument is converted into an electrical signal which may then be electronically processed, amplified and recorded. Such instruments include electric guitars, which typically employ one or more magnetic pickups and acoustic instruments, outfitted with contact microphones and the like. In addition, musical instruments of all descriptions can be suitably played near a microphone to convert the sound into an electrical signal.
In the music and recording industries there is a wide variety of different pickups, transducers and microphones in use today. Where the instrument will be situated a long distance from the electronic processing, amplifying or recording equipment it is common to use a low impedance transmission line or cable system, often a balanced system, to minimize hum and ground loop problems. It is generally recognized that a low impedance transmission line or cable system is less susceptible to magnetically induced hum. Moreover, a balanced system, which conveys both in-phase and out-of-phase signals has the recognized advantage of eliminating common mode noise. Thus low impedance, balanced transmission lines or cables are frequently used in professional music and recording applications.
With the exception of professional low impedance, balanced microphones, most musical instrument pickups and transducers are high impedance, single ended (unbalanced) devices. For example, the classic guitar pickup may have an impedance in the range of about 50K ohms and the more modern guitar pickups with active electronic circuits may have an impedance in the range of 3K-4K ohms. When compared to a low impedance system, which is nominally 300 ohms or less, these guitar pickups are considered to be high impedance devices. Being high impedance devices, these pickups, and the transmission lines or cables connected to them tend to be quite susceptible to hum and noise.
To deal with the hum and noise problem in high impedance instruments it is common practice to use an impedance matching transformer or active electronic circuit to couple the high impedance source to a low impedance destination. Such matching transformers or circuits are sometimes referred to as "direct boxes" since they allow a high impedance device to be connected "directly" to a low impedance mixing console or other low impedance electronic device. Transformers have been employed for this purpose for many years but they are considered undesirable today, since transformers tend to degrade or color the sound by diminishing the high frequencies. Thus, today, many audio engineers prefer using active electronic circuits to match or couple the high impedance audio source to the low impedance audio destination.
To a large extent, active electronic circuits are implemented today using solid state transistor circuitry. There remains, however, a loyal group of audio engineers and musicians who favor vacuum tube circuitry because of its desirable, warm sound. Although the precise physics of this phenomenon are not fully understood, it is believed that the warm sound is attributable to the vacuum tube's tendency to produce even harmonics, which can add a subtle warming effect that is not normally found in solid state circuits. In contrast, solid state circuits may tend to produce a larger number of odd harmonics which to some sound somewhat strident or shrill.
The present invention provides a circuit for coupling a high impedance audio source to a low impedance audio destination which employs at least one solid state operational amplifier and at least one vacuum tube. The operational amplifier and vacuum tube are connected together so that the vacuum tube is in the feedback loop of the operational amplifier. The feedback loop includes a dampening impedance which permits the desirable warming characteristics of the tube to influence the output signal, but without inducing unwanted distortion or tube microphonics. The circuit is thus suitable for use in "direct box" applications particularly where the warm tube characteristic is desired.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawing.